Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with a pathogen, insect feeding, parasitism by another plant such as mistletoe, and grazing by ruminant animals. Abiotic stresses include, for example, excessive or insufficient available water, temperature extremes, synthetic chemicals such as herbicides, and excessive wind. Yet plants survive and often flourish, even under unfavorable conditions, using a variety of internal and external mechanisms for avoiding or tolerating stress. Plants' physiological responses to stress reflect changes in gene expression.
Insufficient water for growth and development of crop plants is a major obstacle to consistent or increased food production worldwide. Population growth, climate change, irrigation-induced soil salinity, and loss of productive agricultural land to development are among the factors contributing to a need for crop plants which can tolerate drought. Drought stress often results in reduced yield. In maize, this yield loss results in large part from plant failure to set and fill seed in the apical portion of the ear, a phenomenon known as tip kernel abortion.
Low temperatures can also reduce crop production. An untimely frost in spring or fall may cause premature tissue death.
Physiologically, the effects of drought and low-temperature stress may be similar, as both result in cellular dehydration. For example, ice formation in the intercellular spaces draws water across the plasma membrane, creating a water deficit within the cell. Thus, improvement of a plant's drought tolerance may improve its cold tolerance as well.
Plants have developed numerous physiological and biochemical strategies to cope with stress. Some of the well characterized proteins involved in the protection of plants from dehydration and other stress damage include molecule chaperones, osmotic adjustment proteins, ion channels, transporters, and antioxidation or detoxification proteins. The expression of these is largely regulated by transcriptional activators. Most transcriptional activators regulate their target gene expression by binding to cognate cis-elements in the promoters of the stress-related genes. More than 30 families of transcriptional activators have been identified in Arabidopsis alone. Two well characterized dehydration stress-related cis-elements bound by transcriptional activators include the drought responsible element (DREB) for CBF transcriptional activators which belong to the AP2/ERF class of transcriptional activators, and the abscisic acid (ABA)-responsive element (ABRE) recognized by the bZIP-domain transcriptional activators. There have been several reports of abiotic stress tolerance conferred using CBF/DREB genes. Recently other AP2/ERF have been reported for drought stress tolerance.
NAC transcriptional activators are encoded by genes present in a wide range of plant species, the name being derived from the NAM (no apical meristem; Souer, et al., (1996) Cell 85:159-170), ATAF1,2 and CUC2 (cup-shaped cotyledon 2; Aida, et al., (1997) Plant Cell 9:841-857) transcriptional activators. Expression patterns of NAC transcriptional activators, and the mutant phenotypes conferred by modulation of their expression, are similar. A highly conserved N-terminal DNA-binding domain has been identified and its structure has been characterized (Ernst, et al., (2004) EMBO Reports 5(3):297-303). The more diverse C-terminal regions comprise transcriptional activation domains (Xie, et al., (2000) Genes Dev. 14:3024-3036; Duval, et al., (2002) Plant Mol. Bio. 50:237-248). NAC transcriptional activators have been shown to interact with numerous genes involved in meristem formation, organ differentiation, auxin signaling, root growth, and in biotic and abiotic stress response (see, review by Olsen, et al., (2005) Trends in Plant Science 10(2):79-87). A NAC recognition sequence (NACRS), and a core binding sequence, have been identified in Arabidopsis (Tran, et al., (2004) Plant Cell 16:2481-2498). At least one report has reported a rice NAC transcriptional activator involved in conferring drought tolerance.
Each different transcriptional activator is capable of turning on only a subclass of downstream genes involved in abiotic stress tolerance and thus regulates only some of the secondary responses to stress. Continued discovery of new transcriptional activators and their regulation to provide tolerance to stress conditions is of interest.